What is the smallest MechMate off the plans? Well, I am very tempted to do a 1200mm [4ft] gantry on a 600mm [2ft] table for making signs at home. In this case I would call the gantry x and the table y.

That gantry would easily transfer to a 2400mm [8ft] table if I want to sell it. I think there comes a point where one considers shortening the table instead of the gantry, since 95% of the MechMate's "engineering" and sweat are in the gantry (and control box) and not in the table. Tables are almost consumable/disposable - unless you are heavily invested in vacuum holddown.